The story opens in a fashionable club where young heir Edgar von Hull falls victim to a mysterious elderly gentleman known as Balling, who manipulates the card game through pure psychological domination, stripping the young man of his fortune. Hull awakens from a trance-like state to find himself deeply in debt and utterly disoriented, initiating a mystery that deepens when the address given by Balling proves false and points to an innocent American tourist.
As underground gambling dens flourish across the city, state attorney von Wenk becomes obsessed with stopping the epidemic of chance that is rotting the nation's core. Wenk investigates the string of anomalous losses and encounters a brilliant dancer connected to the underworld, recognizing the pattern of a single, highly coordinated criminal mind operating behind various disguises.
Meanwhile, the mysterious mastermind—Dr. Mabuse—orchestrates an elaborate transnational conspiracy involving currency smuggling, underground networks, and the psychological subjugation of everyone around him. His ultimate ambition is to accumulate vast wealth to establish an autocratic empire in the primeval forests of Brazil called Citopomar.
Hull becomes increasingly paranoid, convinced he is being watched, and eventually meets a tragic end when he is murdered in the dark streets to silence his cooperation with the authorities. This violent act serves as a brutal warning to anyone who dares to interfere with the master criminal's plans, escalating the stakes for Wenk's investigation.
The narrative follows Wenk as he adopts various disguises and navigates dangerous underground gambling houses to match wits with the elusive adversary. Along the way, Wenk crosses paths with the enigmatic Countess Told, whose aristocratic boredom draws her into the shadowy world of high-stakes play and psychological manipulation.
The net tightens as Wenk traces the criminal's movements from luxury hotels to remote border towns, uncovering an empire of obedient followers bound by absolute terror. The psychological cat-and-mouse game between the relentless investigator and the mastermind reaches a fever pitch as Mabuse balances his grandiose ambitions with personal obsessions, culminating in a tense confrontation.
